diff: a submodule not checked out is not modified

diff: a submodule not checked out is not modified

948dd34 (diff-index: careful when inspecting work tree items, 2008-03-30)
made the work tree check careful not to be fooled by a new directory that
exists at a place the index expects a blob.  For such a change to be a
typechange from blob to submodule, the new directory has to be a
repository.

However, if the index expects a submodule there, we should not insist the
work tree entity to be a repository --- a simple directory that is not a
full fledged repository (even an empty directory would do) should be
considered an unmodified subproject, because that is how a superproject
with a submodule is checked out sparsely by default.

This makes the function check_work_tree_entity() even more careful not to
report a submodule that is not checked out as removed.  It fixes the
recently added test in t4027.

Signed-off-by: Junio C Hamano <gitster@pobox.com>